This invention relates to an improved sol-gel method for preparing silica glass and, in particular, to an improved sol-gel method for preparing high quality silica glass.
Silica glass is useful for many purposes, including glass wafers, semiconductor industrial material, optical material, preforms for optical fibers, support tubes and photomask substrates. Accordingly, the demand for silica glass is expected to expand significantly in coming years.
The sol-gel method has been used as a method for preparing inexpensive silica glass and various sol-gel methods are known. Such methods are described, for example, in the "Journal of Non-Crystalline Solids", Vol. 37, No. 191 (1980) by Nogami et al, "Journal of Non-Crystalline Solids", Vol. 47, No. 435 (1982) by Rabinovitch et al, U.S. patent application Ser. No. 642,606 filed on Aug. 20, 1984 by Toki et al and U.S. patent application Ser. No. 826,527, filed on Feb. 6, 1986 by Matsuo et al.
The differences between these methods arise from differences in the components of the sol solution, which is the main starting material. The methods can be classified into four groups as follows:
1. The sol solution is prepared by hydrolyzing a mixture of a silicon alkoxide, water, alcohol and an appropriate catalyst such as hydrochloric acid, ammonia and the like. This is the method of Nogami et al;
2. The sol solution is prepared by mixing a solution of a silicon alkoxide hydrolyzed with an acid reagent and a solution containing fine particle silica obtained by hydrolyzing a silicon alkoxide with a basic reagent at a predetermined mixing ratio. This is the method of Matsuo et al;
3. The sol solution is prepared by mixing a solution of a silicon alkoxide hydrolyzed with an acid reagent and ultrafine particle silica at a predetermined mixing ratio. This is the method of Toki et al; and
4. The sol solution is prepared by diffusing ultrafine particle silica in water or an organic solvent at a predetermined ratio. This is the method of Rabinovitch et al.
The sol solutions prepared by each of the above methods are gelled in a container of the desired shape and the resulting dry gel is sintered to yield a silica glass article. Each of the methods has distinct advantages and disadvantages. The features of each method are shown in the following table.
TABLE 1 ______________________________________ Nogami Matsuo Toki Rabinovitch ______________________________________ purity of the silica glass .circleincircle. .circleincircle. .DELTA. .DELTA. cost of raw material .largecircle. .DELTA. .largecircle. .circleincircle. yield X .circleincircle. .circleincircle. .largecircle. size of article X .circleincircle. .circleincircle. .DELTA. suitability for X .DELTA. .circleincircle. .largecircle. mass-production ______________________________________ .circleincircle. = excellent .largecircle. = very good .DELTA. = satisfactory X = unsatisfactory
The method of Toki et al is the best from the point of view of productivity. The method of Matsuo et al is the best in terms of physical properties and purity of the resulting glass.
When any of the sol solutions described above are used as a starting material and are dried and sintered to yield a silica glass article, inclusions occur in the resulting article. In order to enhance the quality of the glass, Matsuo et al has succeeded in removing inclusions larger than a few microns by performing the sol-gel procedure for making a silica glass article in a clean environment, irradiating the sol solution with supersonic waves in order to improve the dispersion properties, and filtering and centrifugally separating the sol solution. Matsuo et al have also succeeded in preventing bubbling by closing the pores in the dry gel by sintering the dry gel in a helium atmosphere or under reduced pressure.
According to the method of Matsuo et al, the inclusions in the resulting silica glass articles are reduced remarkably. However, the resulting silica glass articles still include silica crystals, some inclusions, microcracks and bubbles. For this reason, the presently available silica glass cannot be used in fields where extremely high quality is required, specifically for photomask substrates and preforms for optical fibers.
Accordingly, it is desirable to provide an improved method of making silica glass articles which are free of silica crystals, inclusions, microcracks and bubbles which will overcome the shortcomings of the prior art.